Reversible dry adhesives

ABSTRACT

One embodiment of the invention includes product comprising an elastomeric epoxy dry adhesive with a pull-off strength of 1-200 N/cm 2  from a substrate and reversibly detached with a peel-off force less than 1 N/cm.

This application claims the benefit of U.S. Provisional Application No. 60/925,418, filed Apr. 20, 2007.

FIELD OF THE INVENTION

The field to which the disclosure generally relates includes compositions of epoxy dry adhesives.

BACKGROUND

Gecko feet pads, with nanohair structures on them, are examples of smart dry adhesives. The working principle of the Gecko adhesion is that the nanohair structure allows the foot pad to make maximum contact with a counter surface regardless of its roughness and chemical composition. This is accomplished by nanohairs that are relatively long and protruding from the foot pad at an angle so that adjacent nanohairs can contact the counter surface regardless of its topography. The maximum contact further allows for accumulation of millions of small van der Waals (in the range of microNewtons) interactions between the Gecko foot pad and the counter surface, leading to an overall adhesion force (pull-off force) of about 10 N/cm². When the detaching force is employed in a peel-off mode, however, the complete detachment is achieved gradually by overcoming small adhesion forces corresponding to very small areas. Thus, the adhesion is easily reversed. Overall, the attractiveness of the Gecko adhesion lies in the combination of adhesive strength (10 N/cm²), reversibility, and the ability to adapt to a variety of surfaces in terms of both the surface roughness and composition. The above unique features of the Gecko adhesion has stimulated scientific research efforts to produce synthetic smart dry adhesives that work using the same principle as the Gecko feet. Up to now, the two best synthetic Gecko adhesives show maximum pull-off adhesive strength of 3 and 10 N/cm² towards glass. Both adhesives suffer from severe adhesion loss after only one or two attaching/detaching cycles, as a result of breakdown and the lateral collapse of the nano structures, with the latter referring to the adjacent nano hairs of the Gecko foot pad bonding to each other. In addition, synthetic Gecko adhesives are expensive to produce and large-scale manufacturing is practically too difficult.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

One embodiment of the invention includes a dry adhesive that functions similar to a gecko footpad.

One embodiment of the invention includes a product comprising an elastomeric epoxy dry adhesive with a pull-off strength of 1-200 N/cm² from a stainless steel 304 alloy substrate (SS304).

One embodiment of the invention includes a composition comprising an aliphatic diepoxy and a diamine curing agent.

Another embodiment of the invention includes a method comprising curing a composition comprising an aliphatic diepoxy and a diamine curing agent to provide an elastomeric epoxy dry adhesive capable of adhering to a surface with a pull-off strength greater than 10 N/cm² from an SS304 substrate and a peel-off force of 1 N/cm or less from the same substrate.

Another embodiment of the invention includes a method comprising providing an elastomeric epoxy dry adhesive produced from a composition including an aliphatic diepoxy and a diamine curing agent, placing the adhesive on a surface, preloading the adhesive with the force so that the adhesive has a pull-off strength greater than 10 N/cm² from an SS304 substrate, and peeling off the adhesive using a peel-off force of 1 N/cm² or less from the same substrate, and repeating the attaching and peeling off steps more than six times.

Other exemplary embodiments of the invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while disclosing exemplary embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

FIG. 1 illustrates the chemical structures of Jeffamine D-230 and NGDE.

FIG. 2 illustrates a pull-off force on a rigid backing of a dry epoxy adhesive.

FIG. 3 is a graph of pull-off strength versus attaching/detaching cycle number.

FIG. 4 illustrates the peel-off force of a dry adhesive.

FIG. 5 illustrates the chemical structures of EPON 826 and octadecyl amine.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following description of the embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.

One embodiment of the invention includes a dry adhesive with functions similar to a gecko footpad. In one embodiment, the dry adhesive may have a glass transition temperature T_(g) ranging from −90 to 25° C.

One embodiment of the invention includes a product comprising an elastomeric epoxy dry adhesive with a pull-off strength of 1-200 N/cm² from a SS304 substrate. With respect to the pull-off strength, the invention is not limited to a pull-off strength from a SS304 substrate. Various substrates may be used to achieve a pull-off strength of 1-200 N/cm².

One embodiment of the invention includes a composition comprising an aliphatic diepoxy and a diamine curing agent.

Another embodiment of the invention includes a method comprising curing a composition comprising an aliphatic diepoxy and a diamine curing agent to provide an elastomeric epoxy dry adhesive capable of adhering to a surface with pull-off strength greater than 10 N/cm² from an SS304 substrate and a peel-off force of 1 N/cm or less from the same substrate.

Another embodiment of the invention includes a method comprising providing an elastomeric epoxy dry adhesive produced from a composition including an aliphatic diepoxy and a diamine curing agent, placing the adhesive on a surface, preloading the adhesive with the force so that the adhesive has a pull-off strength greater than 10 N/cm² from an SS304 substrate, and peeling off the adhesive using a peel-off force of 1 N/cm or less from the same substrate, and repeating the attaching and peeling off steps more than six times.

In one embodiment of the invention, an elastomeric epoxy dry adhesive is produced. In one embodiment, 4.32 g of neopentyl glycol diglycidyl ether (NGDE) was mixed with 2.3 g of Jeffamine D-230, which is the curing agent poly(propylene glycol)bis(2-aminopropyl)ether. Jeffamine D-230 is a polyetheramine that is difunctional, primary amine with an average molecular weight of about 230. The primary amine groups are located on secondary carbons at the end of the aliphatic polyether chain. NGDE may be obtained from TCI America, and Jeffamine D-230 may be obtained from Huntsman. The chemical structures for Jeffamine D-230 and NGDE are shown in FIG. 1. A liquid mixture of the 4.32 g of NGDE and the 2.3 g of Jeffamine D-230 was poured into an aluminum mold. The mixture was cured in an oven for about 1.5 hours at 100° C. The mixture was then postcured for about 1 hour at 130° C.

The cured epoxy dry adhesive was demolded and cut into pieces of about 1 cm×1 cm. To measure the pull-off force, one side of the 1 cm×1 cm epoxy dry adhesive piece 10 was bonded to a rigid substrate or backing 12 such as a rigid epoxy. The other side of the epoxy dry adhesive piece was placed on a substrate 14, for example SS304. A preload force of 4 N was imposed by applying a load 16 on the rigid substrate surface facing up with the dry epoxy adhesive side facing down to the substrate 14 as shown in FIG. 2.

The adhesive forces were measured with a load cell and the maximum pull-off force corresponding to multiple attaching/detaching cycles for the same sample is shown in FIG. 3. The data in FIG. 3 shows that the pull-off force does not degrade after six attaching/detaching cycles and the average pull-off force was about 60 N/cm².

To measure the peel-off forces, the dry adhesive 10 with no rigid backing was pressed onto the SS304 surface 14 with a preload of 4 N applied by means of the load 16 and the separation force was imposed in a peel-off mode and measured with a load cell as illustrated in FIG. 4. The peel-off force was less than 1 N/cm. In one embodiment, the elastomeric dry adhesive can undergo multiple attaching and detaching cycles without adhesion loss.

In another embodiment, NGDE and Jeffamine D-230 were mixed in the ratios listed in Table 1 below. The mixtures were cured at 100° C. for 1.5 hours and postcured at 130° C. for 1 hour. The resulting pull-off strength at various molar ratios of NGDE to Jeffamine D-230 is shown in Table 1 below.

TABLE 1 Pull-off strength of samples 1-7 Molar ratio of Pull-off strength Sample # NGDE/Jeffamine D-230 (N/cm²) 1 2.12 52.4 2 2.09 49.6 3 2.04 56.1 4 2.00 50.7 5 1.96 60.0 6 1.92 59.0 7 1.89 43.8

In another embodiment, while maintaining the total number of amine groups at twice the total number of epoxy groups, EPON 826 (the diglycidyl ether of bisphenol A epoxy monomer), Jeffamine D-230, and octadecyl amine (CH₃(CH₂)₁₇NH₂, an exemplary aliphatic monoamine) were mixed at different ratios. The chemical structures for EPON 826 and octadecyl amine are shown in FIG. 5. EPON 826 may be obtained from Hexion. The mixtures were cured at 100° C. for 1.5 hours and postcured at 130° C. for 1 hour. When the molar ratio between Jeffamine D-230 and octadecyl amine varied from 0 to infinity, the pull-off strength obtained from the cured epoxies fell within the range of 1 N/cm² to 100 N/cm² from an SS304 substrate.

The amine utilized in the composition according to various embodiments of the invention may be an aliphatic, branched, or aromatic amine.

One embodiment includes a method including curing a composition comprising at least one multi-functional epoxy (either aliphatic or aromatic) and at least one of a curing agent (for example, an aromatic or aliphatic anhydride, or an aromatic or aliphatic multi-amine) or a catalyst, to provide an elastomeric epoxy dry adhesive with a pull-off strength of 1-100 N/cm² from an SS304 substrate.

In various embodiments, the components of an elastomeric epoxy dry adhesive may include a rigid epoxy and a flexible extender. The range of possible crosslinking chemistries which may be used to achieve elastomeric epoxy dry adhesives may include alpha, omega-diaminoalkanes, organic multi-carboxylic acid, anhydride, or catalytic (as in imidazole type) crosslinking reactions. There are many different ways to achieve the appropriate relationships between the molecular properties. For example, the elastomeric epoxy dry adhesive may include a rigid epoxy, an epoxy extender, and a crosslinking agent; or a rigid epoxy, a flexible crosslinking agent, and a flexible epoxy; or a rigid epoxy, a rigid crosslinking agent, and a flexible epoxy; or a rigid epoxy, a flexible epoxy, and a catalytic curing agent; or a rigid epoxy, a crosslinking agent, and a diluent; or a flexible epoxy, a crosslinking agent, and a diluent; or a rigid epoxy and a flexible crosslinking agent; or a flexible epoxy and a catalytic curing agent; or a flexible epoxy and a crosslinking agent; and wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, the epoxy extender has one epoxide group, and the crosslinking agent is one of a multi-amine, an organic multi-carboxylic acid, or an anhydride, and the diluent is a monoamine or a mono-carboxylic acid. In various embodiments, the catalytic curing agent (or catalytic cure) promotes epoxy-to-epoxy or epoxy-to-hydroxyl reactions. The catalytic curing agent may include, but is not limited to, tertiary amines, amine salts, boron trifluoride complexes, or amine borates. The components of the dry adhesive may be present in an amount sufficient to provide, upon curing of the composition, an elastomeric epoxy dry adhesive having a glass transition temperature below room temperature and having a pull-off strength of 1-200 N/cm² from a substrate.

The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention. 

1. An elastomeric epoxy dry adhesive composition comprising: at least one of a rigid epoxy or a flexible epoxy; and at least one of a crosslinking agent or a catalytic curing agent; at least one of an epoxy extender having, one epoxide group, or a diluent comprising a monoamine or a mono-carboxylic acid; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, and the crosslinking agent comprising at least one of a multi-amine, an organic multi-carboxylic acid, or an anhydride; and wherein the components are present in an amount sufficient to provide, upon curing of the composition, an elastomeric epoxy dry adhesive having a glass transition temperature below room temperature and having a pull-off strength of 1-200 N/cm² from a substrate; and when the crosslinking comprises a multi-amine the total number of epoxy groups to amine groups ranges from 1.89-2.12.
 2. An elastomeric epoxy dry adhesive composition as set forth in claim 1 comprising an epoxy extender having one epoxide group.
 3. An elastomeric epoxy dry adhesive composition as set forth in claim 1 comprising a diluent, wherein the diluent is a monoamine or a mono-carboxylic acid.
 4. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the elastomeric epoxy dry adhesive has a pull-off strength of 10-100 N/cm² from a substrate.
 5. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the elastomeric epoxy dry adhesive is capable of adhering to a substrate with a peel-off force less than 1 N/cm from the substrate.
 6. An elastomeric epoxy dry adhesive composition as set forth in claim 1 comprising an aliphatic diepoxy and a diamine.
 7. An elastomeric epoxy dry adhesive composition as set forth in claim 6 further comprising an aromatic diepoxy.
 8. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the aromatic epoxy is the diglycidyl ether of bisphenol A epoxy monomer with an epoxy equivalent weight of
 180. 9. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the aliphatic epoxy is NGDE.
 10. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the multi-amine is poly(propylene glycol)bis(2-aminopropyl)ether with an average molecular weight of
 230. 11. An elastomeric epoxy dry adhesive composition as set forth in claim 3 comprising an aromatic diepoxy, a diamine, and an aliphatic monoamine.
 12. An elastomeric epoxy dry adhesive composition as set forth in claim 1 wherein the flexible epoxy has a molecular weight equal to or greater than
 216. 13. An elastomeric epoxy dry adhesive comprising: the reaction product of at least one of a rigid epoxy or a flexible epoxy, and at least one of a crosslinking agent or a catalytic curing agent; said dry adhesive being cured at about 100° C. and post-cured at about 130° C.; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, and the crosslinking agent is one of a multi-amine, an organic multi-carboxylic acid, or an anhydride; and wherein the elastomeric epoxy dry adhesive has a glass transition temperature below room temperature and has a pull-off strength of 1-200 N/cm² from a substrate; and when the crosslinking comprises a multi-amine the total number of amine groups to epoxy groups ranges from 1.89-2.12.
 14. An elastomeric epoxy dry adhesive as set forth in claim 13 further comprising the reaction product of an epoxy extender having one epoxide group.
 15. An elastomeric epoxy dry adhesive as set forth in claim 13 further comprising the reaction product of a diluent, wherein the diluent is a monoamine or a mono-carboxylic acid.
 16. An elastomeric epoxy dry adhesive composition comprising: at least one of a rigid epoxy or a flexible epoxy; and at least one of a crosslinking agent or a catalytic curing agent; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, and the crosslinking agent comprising a multi-amine, and wherein the total number of epoxy groups to amine groups ranges from 1.89-2.12, and wherein the components are present in an amount sufficient to provide, upon curing of the composition at least 100° C., an elastomeric epoxy dry adhesive having a glass transition temperature below room temperature and having a pull-off strength of 1-200 N/cm² from a substrate.
 17. An elastomeric epoxy dry adhesive comprising: the reaction product of at least one of a rigid epoxy or a flexible epoxy, and at least one of a crosslinking agent and a catalytic curing agent; wherein the rigid epoxy is an aromatic epoxy having at least two epoxide groups, the flexible epoxy is an aliphatic epoxy having at least two epoxide groups, the catalytical curing agent comprises a tertiary amine, amine salt, boron trifluoride complex, or amine borate, and the crosslinking agent comprises a multi-amine, and wherein the total number of epoxy groups is about twice the total number of amine groups; and wherein the elastomeric epoxy dry adhesive has a glass transition temperature below room temperature and has a pull-off strength of 1-200 N/cm² from a substrate. 